1. Field of the Invention
The present invention relates to a voice frequency communication apparatus which applies a voice frequency digital communication system utilizing a low bit rate encoding signal having the transmission rate, for example, of 16 kbps or 9.6 kbps, to the facsimile communication and communication through personal computer.
2. Description of the Prior Art
FIG. 1 is a block diagram of a voice frequency communication apparatus for digital low bit rate transmission which is capable of transmitting a voice frequency terminal signal/non-voice frequency terminal signal of the prior art disclosed, for example, in the Japanese Patent Publication No. 63-1238 (1988) filed to the Patent Office in Japan. In the same figure, the reference numeral 1 designates a voice frequency input/output I/F circuit as an interface circuit for input and output of an analog voice frequency signal in the frequency bandwidth of 300-3400 Hz; 2, an A/D converter for converting the analog voice frequency signal from this voice frequency input/output I/F circuit 1 into the digital signal having the transmission quality equal to or higher than that in the PCM (Pulse Code Modulation) encoding system by .mu.-law (.mu.=255); 3, a D/A converter for converting a digital signal to an analog signal by the decoding system which is equivalent to the A/D converter 2; 4, a 9.6 kbps/16 kbps encoder (hereinafter referred only to as encoder) as a data conversion circuit which compresses the digital signal from the A/D converter 2 into the digital data signal of 9.6 kbps or 16 kbps; 5, a 9.6 kbps/16 kbps decoder (hereinafter referred only to as decoder) as a data conversion circuit for expanding the 9.6 kbps or 16 kbps digital data signal to the digital signal which is more redundant than 16 kbps by the data conversion system which is identical to the encoder 4; 6, a communication line I/F circuit as an interface circuit between a digital communication line (CL) and the present apparatus; 14, a data selector for selecting the signal path with the voice frequency terminal signal and non-voice frequency terminal signal; 36, a demodulator for non-voice frequency terminal; 37, a modulator; 38, a frame forming circuit; 39, a frame separation circuit; 40, an analog switch for switching the signal path with the voice frequency terminal signal and non-voice frequency terminal signal; 41, a non-voice frequency terminal distinguishing circuit for controlling the data selector 14 and the analog switch 40.
Next, operations will be explained In FIG. 1, the signal from the voice frequency terminal is detected as a detection output in the level "0" by the non-voice frequency terminal distinguishing circuit 41 and the data selector 14 and analog switch 40 do not operate and are kept in the connecting condition indicated in the figure. Therefore, the sending signal from the voice-frequency terminal is encoded into a PCM code by the A/D converter 2 and is then compressed by the encoder 4 and is transmitted to the digital communication line through the communication line I/F circuit 6. The receiving digital data signal from the digital communication line is expanded by the decoder 5 through the communication line I/F circuit 6, decoded to the initial voice frequency by the D/A converter and is then supplied to the terminal as the receiving output signal.
Meanwhile, a call is terminated from the non-voice frequency terminal, an identifying signal which has a constant frequency and continues for a certain period is detected by the non-voice frequency terminal distinguishing circuit 41 consisting of a band-pass-filter and a detector and the detected output changes over two data selectors 14 and two analog switches 40. As a result, the transmitting signal from the non-voice frequency terminal is demodulated to the original digital code by the decoder 36, a control bit and a frame bit are added in the frame forming circuit 38 to such transmitting signal, and finally this transmitting signal is sent through matching of transmission rate with the digital communication line. As an example, when a voice frequency encoding speed and digital transmission path speed are set to 16 kbps and data encoding speed from the non-voice frequency terminal is set to 9.6 kbps, matching of speed can be realized by adding total of 4 bits combining the control bit and frame synchronization bit to the data code of 6 bits and then transmitting the signal in the frame format that a frame is formed by 10 bits. Moreover, the digital data signal from the digital communication line enters a frame separation circuit 39 through the communication line I/F circuit 6 and separates the data code and control bit. This signal then enters the modulator 37 to become the analog modulated signal through modulation of the carrier and then sent to the non-voice frequency terminal as the receiving output signal. Operations of the data selector 14 and analog switch 40 are held during transmission of data from the non-voice frequency terminal but are recovered when the communication terminates and the transit line is set free.
A voice frequency communication apparatus which is capable of transmitting the conventional voice frequency terminal signal and non-voice frequence terminal signal is thus constituted as described above. Therefore, it has a problem that when the signal including the frequency component detected by the non-voice frequency signal distinguishing circuit 41, although it may be the signal from the voice frequency terminal, is input for a constant period, the apparatus erroneously recognizes such signal as the signal from the non-voice frequency terminal and changes the signal path to the line for processing non-voice frequency terminal signal utilizing the demodulator and modulator and thereby such voice-frequency terminal signal cannot be transmitted correctly.
Moreover, when the signal such as the modem signal conforming to the CCITT recommendation V.29, which shows a higher bit error rate through the encoder and decoder of the voice frequency communication apparatus, is input, for example, this modem signal of V.29 is output to the digital communication line in the route passing through the encoder and is also input to the non-voice frequency terminal distinguishing circuit as the analog signal. Accordingly, when the detection frequency of the distinguishing circuit is equal to the carrier frequency of the modem signal defined in V.29, the non-voice frequency distinguishing circuit operates, after a predetermined period, the analog switch and data selector in order to change the signal path to the line for processing non-voice frequency terminal utilizing the demodulator and modulator Accordingly, the modem signal of V.29 is processed by the demodulator and frame forming circuit and thereafter output to the digital communication line. Meanwhile, the voice frequency communication apparatus in the receiving side provided opposed to the sending side receives first, as is obvious from operation of the voice frequency communication apparatus in the sending side, the signal obtained by processing the modem signal of V.29 with the encoder, as the data A, from the digital communication line. Moreover, after a predetermined period, it receives the signal processed by the demodulator and frame forming circuit as the data B. As a result, the data A received first by the voice frequency communication apparatus in the receiving side is the D/A converted analog signal expanded by the decoder and this signal is then input to the non-voice frequency terminal distinguishing circuit However, the data B received by the voice frequency communication apparatus in the receiving side after a constant period is identical to a distorted signal including a high bit error rate obtained by encoding and then decoding the modem signal of V.29. In addition, the non-voice frequency terminal distinguishing circuit sometimes cannot detect the carrier frequency of modem signal of V.29 or takes a longer period for detection of such modem signal. Moreover, since changeover is not carried out for the signal processing path until the detection is continued for a predetermined period even after the data A is once detected, if reception of data B is started before passage of such detection period, the data B is processed by the decoder and the output thereof becomes abnormal coded output. The analog signal obtained by D/A conversion of such abnormal signal naturally has abnormal waveform. Accordingly, if it is input to the non-voice frequency terminal distinguishing circuit, this circuit does not decide the input signal as the non-voice frequency terminal signal and the signal processing path is not changed over. Therefore, a problem arises herein, namely the signal processing path is different in the apparatus of the sending and receiving sides and thereby non-voice frequency terminal communication is disabled.
In addition, for the facsimile communication, the CCITT recommendation proposes use of the V.21 modem signal (300 bps) or V.27 per modem signal (2400 bps) as the control signal but transmission of modem signal tends to become more difficult as the bit rate of voice frequency encoding system of encoder/decoder becomes lower. In actual, some voice frequency encoding systems of 16 kbps cannot transmit the V.27 per modem signal (2400 bps). When the bit rate of voice frequency encoding system is further lowered, it may be forecasted that only transmission of the V.21 modem signal (300 bps) becomes difficult. With the process similar to that explained with an example of the V.29 modem signal described above, the facsimile communication conceives a problem that not only a message signal but also a control signal cannot be transmitted.
Meanwhile the voice frequency communication apparatus of the prior art indicated above refers, for operations of data selector and analog switch within the apparatus, only to "that operations are held during communication of non-voice frequency terminal but is restored when communication terminates and the transit line is set free" and does not explain at all how detects the "communicating condition" and "termination of communication", and therefore the voice frequency terminal signal/non-voice frequency terminal signal cannot be transmitted in actual through the digital communication line with the same low bit rate if such detection is unclear.